1. Field of the Invention
The present invention relates to sound field measuring apparatuses and sound field measuring methods which can be applied, for example, to car audio systems. In particular, the present invention relates to a sound field measuring apparatus and a sound field measuring method in which a speaker is driven using a burst signal so that sound is picked up by a microphone, and an output signal from the microphone is filtered using an IIR filter forward and backward along a time axis. This permits measurement of the propagation time of an audio output from a speaker, which is driven by a test signal, with a simple mechanism and high precision even in a high-noise environment.
2. Description of the Related Art
Recently, car audio systems have become available which are capable of providing audio contents with increased sound quality due to techniques such as the use of subwoofers and the correction of various characteristics using digital signal processors.
In regard to such techniques, Japanese Unexamined Utility Model Registration Application Publication No. H6-13292 describes a method in which a speaker is driven by a test signal and the output of the speaker is analyzed so that various characteristics of a sound field are measured. Then, the characteristics are adjusted on the basis of the results of the measurement. Specifically, in Japanese Unexamined Utility Model Registration Application Publication No. H6-13292, a technique is proposed for localizing a sound image with certainty. In this technique, an impulse signal is applied for the test signal, and the signal propagation times from a plurality of speakers to a microphone are measured. On the basis of the measured propagation times, a delay time is set for each speaker output.
Since a car audio system is generally used inside of a vehicle, i.e., a small and closed space, the characteristics of a sound field from the car audio system depend on the vehicle in which the system is installed. Therefore, it is necessary to appropriately adjust the characteristics of the sound field in accordance with vehicles. Thus, car audio systems are generally configured such that the sound field can be appropriately adjusted by user operations. The technique proposed in Japanese Unexamined Utility Model Registration Application Publication No. H6-13292 allows such sound field adjustment operations performed by users with increased precision and reduced complexity, thus enhancing operability.
However, a car audio system is usually used under a condition where much noise is present which originates from an air conditioning equipment, the engine, etc. Therefore, when an impulse signal is applied as the test signal for measuring the signal propagation time from a speaker to a microphone, as described above, the presence of noise can degrade the precision of the propagation time measurement.
As a countermeasure for this problem, a technique can be provided in which a burst signal instead of an impulse signal is used as a test signal. Since a burst signal is a sine wave signal having a single frequency, noise can be removed by filtering processing, which can increase the precision of measurement of the signal propagation time from a speaker to a microphone with reduced noise effect. For such filtering processing, a Finite Impulse Response (FIR) filter or an Infinite Impulse Response (IIR) filter can be used.
However, FIR filters have a disadvantage of complex filter configuration. On the other hand, IIR filters have a disadvantage of causing a phase delay, which can degrade measurement precision when applied in such propagation time measurement.